Explosive primer



United States Patent 3,439,611 EXPLOSIVE PRIMER Arnold Harold Holtzman, Cherry Hill Township, N.J., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed Sept. 13, 1967, Ser. No. 667,410 Int. Cl. F42b 3/02 U.S. Cl. 102-24 4 Claims ABSTRACT OF THE DISCLOSURE An explosive primer especially useful for initiating a layer of explosive used to drive metal layers together to bond them comprising a solid body of detonating explosive of cylindrical form surrounded by a sheath of highervelocity detonating explosive extending from one end of the cylindrical body to a location short of the opposite end, and means located at one end of the cylindrical body for initiating the entire end surface of the sheath substantially simultaneously.

Background of the invention The past few years have witnessed the development and commercial acceptance of metallurgically bonded clad products made by explosion bonding. Techniques for explosion bonding are described in U.S. Patents 3,137,937 and 3,264,731, and in copending, co-assigned U.S. Patent application Ser. No. 503,261, now Patent No. 3,397,444. Briefly, the procedure involves propelling metal layers together with an explosive so as to cause them to collide progressively at a velocity which is below 120% of the sonic velocity of the metal in the cladding system having the highest sonic velocity. Continuous metallurgical bonding of the metal layers is produced. It is often difiicult, however, to achieve strong bonding in the small area of the interface which is directly beneath the location of the initiator for the explosive layer. This difliculty often is associated with a failure to supply an impulse to the metal layer in the vicinity of the initiator which is sufficient to provide the conditions required for bonding from the instant of initial impact, while at the same time avoiding severe damage to the metal layer; and/ or with a failure to eliminate, or at least minimize, impact in gross, i.e., surface-to-surface impact, in the vicinity of the initiator.

Conventional point initiators, e.g., blasting caps, generally have not been used in cladding because they often are inadequate to assure a rapid and reliable initiation, especially of the less-sensitive low-detonation-velocity cladding explosives, and because of the impulse which such initiators at the surface of the explosive layer impart to the metal layer adjacent to the explosive layer is lower in the vicinity of the initiator than in farther-removed areas as a result of the divergence of the detonation front; this deficiency in impulse near the initiator being accentuated by the low detonation velocity and energy output of the cladding explosive in the diverging region. The low impulse tends to cause the metal layers to collide at too small an impact angle and too high a collision velocity for good bonding to occur in the initiator region. Larger initiators located inside the explosive layer help to counteract the deficiency in impulse and provide a more rapid and reliable initiation of the cladding explosive. However, the very small area where surface contacts surface on initial impact remains poorly bonded despite the fact that bonding conditions are achieved starting from the periphery of these surfaces. Previously, the poorly bonded area in the vicinity of the initiator has been eliminated by modifying the metal layer on which the explosive is located, e.g., by using expendable metal lice extension pieces at the initiation point (U.S. Patent 3,258,841), or by blasting cap initiation in line with a convex projection in the metal surface (U.S. Patent 3,140,539). In the interests of greater economy and more general applicability, however, it would be highly desirable to have a means of reducing the poorly bonded area in the vicinity of the initiator in the explosion metal cladding process which does not require the use of additional metals or additional steps in the preparative stage of the cladding operation.

Summary of the invention This invention provides an explosive primer, especially useful in explosion metal bonding which comprises (a) a cylindrical solid body of detonating explosive; (b) a sheath of detonating explosive around the periphery of the cylindrical explosive body and in contact therewith, the explosive sheath having a higher detonation velocity than the cylindrical explosive body and extending from a first end of said body to a line short of the end thereof opposite the first end, the line extending around the periphery of said body in a plane which is normal to the cylindrical axis of said body and closer to said opposite end; and (c) means for initiating the entire end surface of the sheath of explosive at said first end substantially simultaneously, e.g., a disk of detonating explosive in contact with the end surface of the sheath and having an initiating device, e.g., a blasting cap, affixed to the center thereof. Preferably, the detonation velocity of the explosive layer or sheath is about 1.2 to 3 times the detonation velocity of the cylindrical solid explosive body, and the sheath extends from an end of the solid cylinder of explosive to a line on the periphery thereof for a distance which is about 70 to of the distance between the ends of the cylinder.

The term cylindrical solid body denotes herein a solid body in the form of a cylinder or a prism of substantially uniform cross-sectional area. The periphery of the body denotes the lateral or cylindrical surface of a cylinder or the lateral faces of a prism; the ends are the bases of a cylinder or prism and they are substantially normal to the periphery. Cylindrical axis is the axis passing through the ends, and is the longest axis. The terms layer and sheath denote a body having substantially greater dimensions in two directions than in the third or thickness direction, the latter direction being normal to the cylindrical axis. The layer or sheath has substantially uniform thickness and forms a hollow body of which the inner surface conforms to the periphery of the cylindrical solid body.

Description of the drawing The accompanying drawing is a cross-sectional view of an explosive primer of the present invention, illustrating a specific embodiment thereof.

Detailed description of the invention The explosive primer of the present invention comprises a solid body of detonating explosive cylindrical form surrounded by a sheath of higher-velocity detonating explosive extending from one end of the cylindrical body to a location short of the opposite end, the primer including means for initiating the entire end surface of the sheath located at one end of the cylindrical body substantially simultaneously.

In the primer depicted in the accompanying drawing, an explosive charge 1 in the form of a cylinder, e.g., a circular cylinder, having peripheral portions a and b, has around peripheral portion a a layer of higher-detonation-velocity explosive 2 which forms an annular sheath around portion a. At the end of cylindrical charge 1 to which layer 2 extends there is a disk 3 of high-velocity detonating explosive, disk 3 abutting the end of cylinder 1 and layer 2. An electric blasting cap 4 is afiixed to the center of disk 3.

When used in the explosion bonding process described above, the primer of this invention is embedded in the cladding explosive layer in a manner such that the end of the primer opposite the initiation end is adjacent the metal cladding layer. Initiation of the sheath explosive in the primer efiects the initiation of the surrounding cladding explosive as well as of the core explosive in the primer, the detonation in the core and sheath travelling in the direction of the cylindrical axis from the initiation end of the primer. The differential in core and sheath detonation velocities causes the detonation in the outer layer to travel ahead of the detonation in the core so that the detonation front impacts against the adjacent metal layer, i.e., the cladding layer, in the bonding assembly as a ring or annulus. This results in a slight inward motion of the metal layer toward the center and subsequent impact from the detonation of the central portion or core of the primer drives the central portion of the metal layer ahead causing a conical deformation of the metal layer. As a result, at the instant of initial impact of the deformed metal layer with the metal layer to which it is to be bonded, the portion of the cladding layer beneath the primer which contacts the other metal layer is the tip of a cone, a sharper cone apex angle, i.e., smaller surface area of the cladding layer at the instant of initial impact, giving a higher degree of bonding. When the initial contacting portion of the cladding layer is a single pont, complete bonding is achieved in the metal layers at the primer axis. Since the highvelocity detonating explosive sheath extends to a location short of the end of the primer opposite the initiation end, i.e., short of the end placed adjacent the cladding layer in the bonding process, the desired metal shaping effect" resulting from the convergent action of the dual-velocity primer is achieved without the metal damaging effect of high-intensity shock waves associated with high-detonation-velocity explosives.

The attainment of good bonding and minimal metal layer damage requires that; the primer be strong enough to drive the cladding layer at a high enough velocity to produce a sufficiently large impact angle and low enough collision velocity as required for bonding, but at the same time that the subjection of the metal layers to strong shock waves be avoided. Consequently, the major constituent of the primer, i.e., the core, is made of an explosive which detonates at a moderately high velocity, e.g., in the range of about from 3000 to 7000 meters per second. Since the detonation velocity of the sheath explosive is higher than that of the core, preferably 1.2 to 3 times higher, the amount of this explosive is kept to a minimum and the sheath usually is quite thin relative to the total cross-sectional area of the primer. The specific dimensions of the primer vary depending on such factors as the properties of the cladding explosive and primer explosives, the mass of the metal layer to be driven, the yield strengths of the metals in the system, and the impact angle and collision velocity desired in the vicinity of the primer. To achieve a desired collision geometry in a given system, larger charges are required with weaker primer explosives, larger driven metal masses, and higher yield-strength metals. Larger charges are required to produce larger impact angles. The length of the primer, i.e., the distance between the cylindrical ends, in general is substantially equal to the thickness of the cladding explosive layer which it is to initiate. Preferably. the cross-sectional 'area of the primer core is the smallest possible which gives the required acceleration to the metal layer without causing damage. As a rule, the diameter or longest cross-sectional dimension, i.e., dimension perpendicular to the cylindrical axis, of the core is greater than 0.5 inch, or the cross-sectional area is greater than 0.25 square inch. Generally, the

longest cross-sectional dimension of the core is at least about one and one-half times, and less than about five times, the thickness of the metal layer adjacent thereto, the larger primers being used with higher yield-strength metals.

As stated previously, the sheath explosive layer, because of its high detonation velocity, is quite thin relative to the total cross-section of the primer. Usually the thickness of the sheats is about from 2 to 15% of the total diameter or longest cross-sectional dimension (core plus sheath). In most cases, the sheath thickness is about 0.02 to 0.2 inch.

The sheath extends from one end of the cylindrical core, i.e., the initiation end of the primer, to a line on the periphery of the core which is in a plane normal to the cylindrical axis and closer to the end opposite the initiation end but short of said opposite end. As a result, the strong shock waves produced by the high-detonation-velocity sheath explosive are attenuated while the convergent action described above is still produced. The length of the sheath in the direction of the cylindrical axis is greater than 50%, and preferably is at least about of the length of the cylinder. In most cases, the length of the portion of the core periphery having no sheath around it, i.e., portion b in the figure, is at least about 10% of the length of the cylinder. However, since the length of this portion which can be used depends in each case on the strength and thickness of the metal cladding layer with which the primer is to be employed, portion b may be less than 10% of the cylinder length, especially with thicker, stronger metals, and longer primers. As a rule, this portion is about 0.25 to 0.75 inch long.

As stated previously, the core explosive is one which detonates at a moderately high velocity, e.g., from 3000 to 7000 meters per second. Typical of the explosives which can be used are PETN, TNT, RDX, HMX, and other organic nitrates, nitramines, and nitro compounds, inorganic azides, and mixtures of the foregoing alone or with other materials as in cast pentolite and /20 amatol. The explosive can be self-supporting, e.g., a cast or plastic explosive, or a powdered explosive maintained in a weak container, e.g., cardboard.

The sheath explosive can be any explosive which detonates at a higher velocity than the core explosive, preferably 1.2 to 3 times higher, is capable of propagating a detonation reliably in the thin-layer fonm employed, and can initiate the lower-detonation-velocity cladding explosive and the core explosive used. Although cast or powdered explosives can be used, self-supporting sheet explosives, e.g., PETN or RDX sheet explosives, are preferred since they can be applied easily by Wrapping around the proper portion of the core periphery.

The peripheral portion of the core having no explosive sheath around it, i.e., portion b in the figure, may either be a free surface or can be surrounded by an inert material such as paper, cardboand, or plastic, so as to form an inert sheath in flush fit with the explosive sheath.

The explosive in the primer is initiated in a manner such that the entire end surface of the explosive sheath at the one end of the primer, e.g., :an annular surface, is initiated substantially simultaneously. This can be achieved conveniently and economically by placing a disk of a detonating explosive capable of initiating the sheath explosive, e.g., a disk of the sheath explosive itself, in contact with the end surface of the sheath, and initiating the disk at its center by means of an initiating device such as a blasting cap. The disk may initiate the end surface of the sheath explosive alone, or it may also initiate the end surface of the core explosive in contact therewith. If the latter occurs, the detonation along the axis of the primer may lead that in the sheath at the beginning, but owing to the differential in detonation velocities of the sheath and core explosives the detonation in the sheath soon leads. If it is desired to delay the initiation of the core until the sheath begins a dentonate, a block of inert material can be placed between the core and an initiating disk, and the sheath extended up to meet the disk; or an inert block can be placed on the core and the sheath extended up and around the block to form an initiating disk. Alternatively, a line wave generator can be used to initiate the sheath at its end surface.

The following example serves to illustrate specific embodiments of the primer of the present invention and its use in explosion bonding. However, it will be understood to be illustrative only and not as limiting the invention in any manner.

Example An explosive primer such as that depicted in the figure has the following specific component structure: The core explosive 1 is a Z-inch-long, l-inchdiameter circular cylinder made of cast trinitrotoluene detonating at a velocity of 5500 meters per second. Sheat 2 is a 0.05-inch-thick layer of a sheet explosive comprised of PETN in an organic rubber and a thermoplastic terpene hydrocarbon resin binder and detonating at a velocity of 6900 meters per second. Peripheral portion a, or the length of the sheath, is 1.75 inches. Disk 3 is a 0.05-inch-thick, 1.1-inchdiameter disk of the same sheet explosive as the sheath. A No. 6 electric blasting cap is placed at the center of the disk.

Two of the above-described primers are employed in the following cladding assembly;

A 12-inch x 10-inch, 0.25-inch-thick 80/20 silver/ copper alloy cladder plate is positioned on each side of a 12-inch x 10-inch, 1.25-inch-thick 80/20 copper/silver alloy backer plate with the plate surfaces substantially parallel to each other and facing surfaces at a standoff of 0.125 inch. One of the primers is placed on the outside surface of each cladder plate at the center of one of the 10-inch sides of the rectangular surface. The positions of the primers on the cladder plates are corresponding positions. The primer is placed on the plate with the end opposite the initiation end contacting the plate and abutting the plate edge. A Wooden frame is placed around the outer surface of each cladder plate, and a granular explosive composition comprised of 9% trinitrotoulene, 35% ammonium nitrate, and 55% sodium chloride is packed into the frame so as to form a Z-inch-thick layer detonating at a velocity of 2000-2400 meters per second. The granular explosive surrounds the primer completely, the frame extending beyond the edge of the cladder plate at the initiator side by 1 inch.

The blasting caps are actuated simultaneously, initiating the pruners which, in turn, 1n1t1ate the granular explosive adjacent thereto as the detonation progresses through the explosive sheath. Initiation of the granular explosive coupled with the impulse from the primers causes the cladder plates to collide progressively with the backer. Examination of the clad product reveals that the plates are bonded in the .area beneath the primers. Framing camera measurements made on the same cladder plate and with the same primer show that steady-state collision conditions are set up within the borders of the primer.

I claim:

1. An explosive primer comprising (a) a cylindrical solid body of detonating explosive of substantially uniform cross-sectional area normal to the cylindrical axis of said cylindrical body;

(b) a sheath of detonating explosive around the periphery of said cylindrical explosive body and in contact therewith, said explosive sheath having a higher detonation velocity than said cylindrical explosive body and extending from a first end of said body to a line short of the end thereof opposite said first end, said line extending around the periphery of said body in a plane which is normal to the cylindrical axis of said body and closer to said opposite end, the cross-sectional area of explosive material, normal to said cylindrical axis, at the portion of said primer extending from said line to said end opposite said first end being equal to said cross-sectional area of said cylindrcial body; and

(c) means for initiating the entire end surface of said sheath of explosive at said first end substantially simultaneously.

2. A primer of claim 1 wherein the detonation velocity of said explosive sheath is about 1.2 to 3 times the detonation velocity of said cylindrical explosive body.

3. A primer of claim 1 wherein the distance between said first end of said cylindrical body and said line extending around the periphery thereof is about from to of the distance between the ends of said body.

4. A primer of claim 1 wherein said initiating means comprise (a) a disk of high-velocity detonating explosive in contact with said end surface of said explosive sheath, and (b) a blasting cap in the center of said disk.

References Cited UNITED STATES PATENTS 2,628,559 2/1953 Jasse 102-24 2,763,210 9/1956 Church et al. 102-24 3,082,689 3/ 1963 Griffith et al. 102-24 3,254,564 6/ 1966 Morley et al. 89-8 3,280,743 10/ 1966 Reuther 102-24 I FOREIGN PATENTS 864,238 3/ 1961 Great Britain.

VERLIN R. PENDEGRASS, Primary Examiner. 

